A piston ring is a small metal piece that fits around the circumference of a piston. It provides a seal between the piston and the cylinder wall that allows the piston to capture all the energy of the expanding gases generated in the engine. Piston rings wear down over time, becoming abraded by the side of the cylinder because of insufficient lubrication or badly formed linings. There are several indicators of worn piston rings.
If your engine is using up motor oil at an unusual rate, this may be a sign of a worn piston ring. Worn rings admit oil into the combustion chamber and burn it. As the engine replaces the burnt oil with fresh supplies, its reserve dwindles. Check this by regularly checking your oil dipstick.
Black smoke coming out of your vehicle's tailpipe is another telltale sign of worn piston rings. The smoke is formed by the burning of motor oil in a combustion chamber designed for gasoline.
Smoke From the Engine Stack
In some cases, a worn piston will cause burnt oil smoke to escape from the engine and out from under the hood. Any sign of smoke is an indication that the vehicle should be taken to the shop; worn pistons may be the cause of other more serious problems.
Loss of Power
A worn piston ring will not capture all the energy of the gases in the cylinder, resulting in a loss of engine power. Pay close attention to the your vehicle's response as you step on the accelerator; worn piston rings will produce a noticeable decline in "get-up-and-go" that will be especially obvious when your vehicle is heavily loaded or towing a trailer.
Reduced Fuel Economy
Worn piston rings cause engine blowby as gases escape from the combustion chamber. This will result in the vehicle needing more fuel to reach and maintain a given speed. A gradual worsening of fuel economy could be an indicator that this is happening and that the piston rings are worn.